1. Field of the Invention
The present invention relates to a pushbutton switch and more particularly to, a pushbutton switch construction for use in a keyboard.
2. Description of the Prior Art
A keyboard as a data entry device for use with a microcomputer, generally has a plurality of pushbutton switches arranged on the front panel thereof. The pushbutton switches are arranged on a membrane sheet which comprises an array of contact circuits allocated by a known printed circuit technique and disposed together with a spacer between two synthetic resin layers sheets so that when the two layers are depressed against each other, a corresponding number of the contact circuits are closed. For each pushbutton switch, an elastic material such as a rubber spring having an inverted bowl shape is placed on the contact circuit of the membrane sheet. When the rubber spring is pressed down by the click action of a key, the circuit closes.
However, the elastic material of such a conventional pushbutton switch has a relatively long stroke for switching action. This results in an increase of the height of the pushbutton switch. For eliminating the above drawback, a modified pushbutton switch has been disclosed in Japanese Patent Laid-open Publication 4-2484432 (1992) or U.S. Pat. No. 5,203,448 where an intermediate element is disposed between the contact region of a key and the substrate on a membrane sheet so that it can slide along the key contact region and the substrate. In action, when the key is depressed, its contact region comes into direct contact with and lowers the intermediate element. The switching stroke of the key is equal to a sum of the clearance between the key and the intermediate element and the clearance between the intermediate element and the substrate. Accordingly, even if the switching stroke of the elastic material is set identical to that of the conventional pushbutton switch, the overall height of the modified pushbutton switch, i.e. from the membrane sheet to the top of the key, can be decreased.
The modified pushbutton switch has the intermediate element mounted between the substrate of the membrane sheet and the contact region of the key so that it can slide along both the substrate and the key contact region and will thus be decreased in overall height. The disadvantage of the modified pushbutton switch is that dimensional error on the sliding element is as large as two times that of the conventional switch because two sliding actions are involved between the key and the intermediate element and between the intermediate element and the substrate. This will cause the vertical movement of the key to wobble more or less and to attenuate the stability. Also, the key is supported by the intermediate element which is not anchored to the substrate or stationary base, thus failing to contribute to the structural strength of the pushbutton switch.
The present invention is directed towards eliminating the above disadvantages and its object is to provide an improved pushbutton switch for opening and closing a contact circuit by way of the resiliency of an elastic material actuated through an intermediate element by a key, in which the switching action of the key is made stable and ensured by the physical strength of an inventive arrangement including the intermediate element and the elastic material on a membrane sheet.